Lithium-ion battery cells are used as traction batteries for vehicles which have an electric drive. A plurality of lithium-ion battery cells are connected to one another to form a battery module. The individual lithium-ion battery cells comprise at least one electrode which can intercalate lithium ions (Li+) or deintercalate said lithium ions again.
DE 10 2008 063 136 A1 discloses a method and an apparatus for protecting a lithium-ion battery in the event of thermal and/or electrical overload as a result of a malfunction, for example a short circuit or overcharging. The solution in accordance with DE 10 2008 063 136 A1 can be used in a hybrid vehicle or an electric vehicle. Battery electronics monitor the operating state of the lithium-ion battery in respect of the current value, the voltage value, the temperature and the cell internal pressure. In the event that stored limit values are exceeded, the lithium-ion battery is disconnected from the electrical distribution system of the vehicle by means of a disconnecting device. The disconnecting device is arranged between the lithium-ion battery and the electrical distribution system, wherein the disconnecting device can be in the form of a relay, reversible fuse or pyrotechnic switch. The disconnecting device is integrated in a current path, wherein, on activation, the current path and therefore the current flow are interrupted.
DE 10 2010 049 611 A1 discloses a battery module unit for vehicles comprising a multiplicity of series-connected individual battery cells, which are separated into submodules in the event of a fault at predetermined desired breaking points by a safety device, wherein the system voltage of said battery module unit is below a critical voltage. The safety device comprises a multiplicity of identical or differently configured disconnecting means which are activated by a control unit, wherein mechanical damage to at least one individual cell is detected directly or indirectly by virtue of variables acting as an indication of this, for example by a crash or airbag sensor.
DE 10 2010 036 379 A1 relates to a high-voltage battery for hybrid or electric vehicles, comprising battery modules which are connected to one another, wherein disconnecting means are provided which are arranged outside an electrical connection in the normal operating mode. As a result, no additional electrical resistance is represented. An accident detected by accident sensors initiates activation of the disconnecting means.
In the context of lithium-ion battery cell technology, the overcharge response of the lithium-ion battery cells, or of the completely interconnected lithium-ion battery, in this context battery module can also be referred to here, has proved to be critical. If lithium-ion battery cell are charged beyond the maximum permissible voltage, this results in damage to the cell even as far as an operation which is also referred to as thermal runaway. In the event of such thermal runaway of the battery cell, it may arise that the cell opens or even that the battery cell bursts or explodes. Such thermal runaway of the battery cell represents the greatest assumed accident in connection with lithium-ion battery cells.
The overcharge operation is stopped by safety devices such as, for example, fuses, since the voltage drop or the short circuit occurring is identified by the battery management system of the battery or the charging operation is interrupted by the short-circuiting of the charger. If, however, the current continues to flow, there is the risk of the material of the membrane of the overcharge safety device (OSD) fusing. In this case, current flows into the battery cell again, or a new galvanic element is formed, comprising what was previously the anode against the housing, manufactured from aluminum, of the battery cell. This in turn enables the occurrence of voltage flashovers or electrolysis, in which a growth of dendrites occurs in the interior of the battery cell. Depending on the voltage level, this results directly in an arc; there is the further possibility of a short circuit forming and the reaction starting again and possibly resulting in a fire. The shielding of the electrodes of the electrode assembly with respect to one another is only possible with difficulty since the entire cell housing acts as an electrode.
Even in the event of the occurrence of an external short circuit, the previously used fuse trips and thus disconnects the electrode assembly. In this case, a new galvanic element, i.e. comprising what was previously the anode against the battery cell housing manufactured from aluminum, is formed directly. The voltage level of the remaining, possibly still intact battery cells is now present there, which results in the same response as that described above. Experiments which were aimed at integrating the fuse in the negative collector, which is generally manufactured from copper, have failed.